The present invention generally relates to a digital telephone set having an emergency switching function, and a communication system having the same.
Recently, there has been considerable activity in the development of a private branch exchange system and terminal equipment coupled thereto, such as a digital telephone set. An advanced private branch exchange system is a digitized system. A digital telephone set is capable of providing various functions and services. Digital telephone sets coupled to a private branch exchange are supplied with power therefrom.
Generally, such a private branch exchange system is provided with an emergency switching device and an analog telephone set. Normally, an analog telephone set is coupled, through the emergency switching device, to an office trunk accommodated by a switch of the private branch exchange. When the private branch exchange stops operating due to a fault for example, the private branch exchange stops supplying the digital telephone sets with power. At this time, the analog telephone set is directly coupled to a public telephone line through the emergency switching system so as to directly communicate with an external telephone set outside of the private branch exchange system. That is, a subscriber (internal telephone user) can send the external telephone set a call or can receive a call sent therefrom.
FIG. 1 is a block diagram of a conventional private branch exchange system having an emergency switching device. Referring to FIG. 1, a digital telephone set 1 is accommodated by a digital subscriber circuit 7 connected to a PBX switch 3 of a private branch exchange PBX through a digital transmission line 6a. The digital telephone set 1 is supplied with power (direct current voltage) from the digital subscriber circuit 7 and serves as an extension telephone. An analog telephone set 2 is accommodated, through an analog transmission line 6b, by an emergency switching device 4 which is provided in the system along with the private branch exchange PBX.
The analog telephone set 2 is selectively connected, through the emergency switching device 4, to an office trunk 9 connected to the PBX switch 3. The emergency switching device 4 includes a first connecting path 4c, a second connecting path 4d, a third connecting path 4e, and two switching relays 4a and 4b. The first connecting path 4c connects a public telephone line 5 and the office trunk 9. The second connecting path 4d connects the public telephone line 5 and the analog transmission line 6b (the analog telephone set 2). The third connecting path 4e connects an analog type subscriber circuit (not shown) and the analog transmission line 6b. The switching relay 4a has two contacts 4a.sub.1 and 4a.sub.2, and the switching relay 4b has two contacts 4b.sub.1 and 4b.sub.2. The switching relays 4a and 4b are connected to ground, and on the other hand to a system operation supervisory control device 8 (hereinafter simply supervisory device 8) through a power line 6c. When the private branch exchange PBX is operating, a contact 8a provided in the supervisory device 8 is set in the closed (conducting) state so that a direct current voltage of -48 V (supervisory signal) is supplied to the switching relays 4a and 4b from the supervisory device 8 through the power line 6c.
While the switching relays 4a and 4b are supplied with a voltage of -48V, the public telephone line 5 and the office trunk 9 are mutually connected through the contacts 4a.sub.2 and 4b.sub.2 of the switching relays 4a and 4b. Further the second Connecting line 4d is set disconnected from the analog transmission line 6b by through the contacts 4a.sub.1 and 4b.sub.1 so that the analog telephone set 2 is disconnected from the office trunk 9. In this state, a terminating call from the public telephone line 5 is supplied to the office trunk 9 through the contacts 4a.sub.2 and 4b.sub.2. Then a predetermined terminating call procedure is subjected to the terminating call. For example, the terminating call is accepted by a public telephone line terminating board (not shown) connected to the PBX switch 3.
On the other hand, when the private branch system PBX stops operating due to the occurrence of a fault for example, the contact 8a of the supervisory device 8 is made open so that no power is supplied to the switching relays 4a and 4b through the signal line 6c. Thereby the switching relays 4a and 4b return to the original state. That is, the first connecting path 4c is disconnected from the public telephone line 5, and the second connecting path 4d is connected to the analog transmission line 6b. Normally, when a failure occurs in the private branch exchange PBX, no power is supplied with the digital telephone set 1 so that it stops operating.
An emergency switching device such as the aforementioned emergency switching device 4 is described in a paper: R. Sugioka et al., "INTRODUCTION OF DIGITAL PBX", Ohm corporation, Sep. 25, 1987, pp.156-159.
However, the private branch exchange system shown in FIG. 1 has the following disadvantages. First, the system is not economical because the analog telephone set 2 must be provided separately from the digital telephone set 1. Second, in a case where the analog telephone set 2 is located away from the digital telephone set 1, it is impossible to let the subscriber on the side of the digital telephone set 1 know the arrival of a call through the analog telephone set 2 which is made active in an emergency.